Use of Chlorine Dioxide and Ozone for Control of Disinfection By-Products

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Control of disinfection by-products (DBPs) in drinking water is becoming increasingly critical as regulatory requirements are calling for higher levels of disinfection while at the same time mandating lower maximum contaminant levels of DBPs. Many utilities have switched to alternative disinfectants such as ozone and chlorine dioxide (ClO2) to minimize chlorinated DBPs. Contra Costa Water District undertook a full-scale demonstration study using ClO2 and ozone to evaluate the potential benefits of combining these two powerful oxidants. The principal research objective, stated broadly, was to evaluate the benefits of using ClO2 in combination with ozone. The specific objectives were to determine the extent to which ClO2 preoxidation of the raw water (1) reduced the ozone demand/dose, (2) reduced the bromate formation, (3) reduced the energy consumption, (4) resulted in chlorate ion formation from oxidation of chlorite ion, (5) maintained the filtration benefits of preozonation, and (6) reduced THM and HAA formation potentials. The full-scale demonstration was conducted at the 40 mgd Randall-Bold WTP, which is a direct filtration plant that uses preozonation (before coagulation) to enhance the filtration process and postozonation (after filtration) for primary disinfection. During the study, ClO2 was added to the raw water ahead of the preozonation contactor. The testing component of the study was divided into three phases that led to long-term full-scale operation using ClO2 in combination with ozone. Three preoxidation conditions were tested: 1.5 mg/L of ClO2 alone, the combination of 1.0 mg/L of ClO2 and 0.5 mg/L of ozone, and 1.0 mg/L of ozone alone. In addition, special full-scale testing was conducted to evaluate bromate mitigation techniques such as preoxidation with ClO2, pH adjustment, and ammonia addition. The data showed that preoxidation with ClO2 substantially reduced the bromate formation. In addition to reducing the bromate formation by reducing the ozone dose through substitution with ClO2, an unexpected result was that ClO2 preoxidation also reduced the bromate formation at a constant ozone dose. Special testing showed that chlorite, which is formed after ClO2 addition, plays an important role in the bromate formation reduction. Preoxidation with ClO2 and pH adjustment to a value of 6.0 were effective methods to reduce the bromate formation in the preozonation contactor, while ammonia addition was less effective. Using ClO2 with ozone resulted in a modest reduction in the energy consumption at the Randall-Bold WTP. The data from this study showed that ozone oxidized the chlorite to chlorate and the GAC filters removed chlorite, so that the plant effluent chlorite concentration was below the detection limit. Preoxidation with ClO2 alone or ClO2 in combination with ozone required similar coagulant dosages and produced similar filter effluent turbidity when compared to using ozone alone. The data showed that there was not a substantial difference in DBP formation potential ClO2 preoxidation. For utilities that use ozone, preoxidation with ClO2 may offer an alternative oxidant that helps meet the treatment goals achieved by oxidation, such as enhanced filtration and DBP formation reduction, while minimizing bromate formation. For utilities that are considering ozonation, preoxidation with ClO2 should be incorporated into the testing protocol for the evaluation. Preoxidation with ClO2 can reduce the required ozonation dose, which can translate into smaller ozonation systems and lower capital costs. Utilities using ozone should consider conducting studies to optimize the ozone generation and ozone dose, which can reduce operating costs and overall energy use. Originally published by AwwaRF for its subscribers in 2004. This publication can also be purchased and downloaded via Pay Per View on Water Intelligence Online - click on the Pay Per View icon below